A rotary encoder is a sensor that detects a position or rotation speed by converting a mechanical displacement amount attributable to rotation into an electrical signal, and by processing the signal.
Rotary encoders are broadly categorized into an incremental type and an absolute type. Generally, the incremental rotary encoders output a signal of two phases called an A phase and a B phase. Generally, this phase difference is a 90-degree electrical angle. A direction of rotation (normal rotation and reverse rotation) is detectable using such a phase relation.
The incremental rotary encoders are categorized into a type (rectangular wave incremental encoder) in which the A phase signal and the B phase signal are rectangular waves, and a type (SIN/COS incremental encoder) in which the A phase signal and the B phase signal are sinusoidal waves. The rectangular wave incremental encoder is a rotary encoder of the type that outputs a rectangular wave (pulse signal) according to a rotational displacement amount of a shaft. Some products among the rectangular wave incremental encoders also output a rectangular wave signal called a Z phase (an output per one rotation) in addition to the A phase and the B phase. The number of pulses output from the rotary encoder is counted by a counter. The amount of rotation is detected from the counted value of the counter.
Using the above-described features, the rotary encoder generates information of devices, for example, an automatic assembly machine, such as speed, moving displacement amount, acceleration, moving direction, etc. The user can control a device or detect various states based on the information.
Inside such a rotary encoder, abnormalities, described below are likely:
(1) Short-circuit in an output circuit to output an A phase signal and a B phase signal;
(2) Disconnection in the output circuit to output the A phase signal and the B phase signal;
(3) Fixation of the logic of either or both of the A phase signal and the B phase signal at either “H” or “L”; and
(4) Abnormality in a detection system (for example, un-flashing of a light projecting element, or the like).
When a certain abnormality of the internal abnormalities has occurred, the output of the rotary encoder indicates that a device stands still even though the device is operating. In such a case, although the device is operating actually, a worker is likely to determine that the device is standing still, from the output of the rotary encoder. Therefore, in order to secure worker's safety, there is the demand for detecting the internal abnormalities of the rotary encoder.
For example, there have been disclosed SIN/COS incremental encoders (refer to “Incremental Encoders” [online], searched on Feb. 3, 2012, Internet <URL:http://www.kuebler.cn/PDFs/leaflet/drehgeber/chinese/5814-5834_SIL_cn.pdf> (hereinafter referred to as Non-Patent Document 1); and “Incremental rotary encoder” [online], searched on Feb. 3, 2012, Internet <URL:http://files.pepperl-fuchs.com/selector_files/navi/productl nfo/edb/t41131_eng.pdf> (hereinafter referred to as Non-Patent Document 2)). Further, there have been disclosed ICs (semiconductor integrated circuits), which diagnose an abnormality of an output signal of such an SIN/COS incremental encoder (refer to “iC-RC1000 SIN/COS SIGNAL SAFETY MONITOR IC”, [online], searched on Feb. 3, 2012, Internet <URL: http://www.ichaus.de/upload/pdf/RC1000_datasheet_A2en.pdf>, <URL:http://www.ichaus.de/upload/pdf/RC1000_flyer_rev1.pdf> (hereinafter referred to as Non-Patent Document 3); and “iC-MSBSAFETY, iC-MSB2 SIN/COS SIGNAL CONDITIONER WITH 1Vpp DRIVER” [online], searched on Feb. 3, 2012, Internet <URL:http://www.ichaus.de/upload/pdf/MSB_datasheet_D2en.pdf> (hereinafter referred to as Non-Patent Document 4). With regard to the SIN/COS incremental encoder, the internal abnormalities of the above-mentioned items (1) to (4) are detectable through known signal processing which uses the theorem “Sin2 θ+Cos2 θ=1”.
However, the rotary encoders disclosed in Non-Patent Documents 1 and 2 output a signal used for detecting an internal abnormality of the concerned encoder. Therefore, to detect the internal abnormality of an encoder, a user has to build a circuit for detection and has to operate the circuit. Therefore, the user's burden increases.
In addition, when such an abnormality detecting device is built by the user-side, the abnormality detecting device serves as a functional safety-related unit. In order to make the abnormality detecting device built by the user to comply with safety-related standard, the device needs to be certificated by allowing a third party standard certificate authority to examine the device. From this point of view, the user's burden further increases.
In addition, to locate or measure a position or speed of rotation with the SIN/COS incremental encoder, it is necessary to process a changing SIN/COS wave into a rectangular wave outside of the encoder. In order to eliminate such signal processing outside of the encoder, a user may consider choosing the rectangular wave incremental encoder. However, Non-Patent Documents 1 to 4 do not disclose any rectangular wave incremental encoders.